16.1 What Are Plants?

16.1 What Are Plants? You have many reasons to be thankful for. Your breakfast came from. In fact, most of your food comes from or from animals that eat. The paper in this book contains wood pulp from. Some of the oxygen you breathe comes from. So the next time you see a plant, be sure to say thanks! In this section you will learn about the characteristics and types of. Plant characteristics Plants vary in size and shape Characteristics common to all Plants come in all sizes, from the tiny duckweed which grows to only about 10 mm in length, to the giant redwood which grows to about 100 m in height. Plants also come in many different shapes like a feathery fern or a prickly cactus. Some examples of are shown in Figure 16.1. Despite their great diversity, all share the following characteristics: Plants are producers and use photosynthesis to make food. Most are green. This is because they contain the pigment chlorophyll. As you read in Chapter 8, chlorophyll absorbs certain wavelengths of light and uses that energy to make carbohydrate molecules. Plants have eukaryotic cells with cell walls. Plant cells have a true nucleus and are surrounded by a cell wall. The cell wall surrounds the cell membrane, protecting the plant and providing a rigid structure. Plants have a cuticle. A cuticle is a waxy layer that covers the parts of a plant that are exposed to air like leaves and stems. The cuticle is an adaptation for living on land that keeps from drying out. Figure 16.1: Some representative. cuticle - a waxy layer that covers the parts of a plant that are exposed to air like leaves and stems. 322

CHAPTER 16: PLANTS Plant classification Vascular and non-vascular Non-vascular Vascular Classification in the Kingdom Plantae is based on the presence or absence of vascular tissues. Vascular tissues are made of cells organized into tube-like structures that transport water, minerals, and food throughout a plant. Non-vascular do not have any tissues to transport water and nutrients. Instead, they depend on the processes of diffusion and osmosis to supply their cells with nutrients. Because these processes are slow, non-vascular cannot grow very tall. Mosses and liverworts are examples of non-vascular. Vascular have tissues made of cells that transport water and nutrients throughout the plant. Like your veins and arteries, vascular tissues can transport materials over a distance. The evolution of vascular tissues is one of the adaptations that allowed to move onto land. Vascular are divided into two groups those that produce seeds and those that do not. Plants that do not produce seeds include ferns, club mosses, and horsetails. Plants that produce seeds are divided into gymnosperms and angiosperms, which you ll learn more about later. vascular tissues - cells organized into tube-like structures that transport water, minerals, and food throughout a plant. non-vascular - do not have any tissues to transport water and nutrients. vascular - have tissues made of cells that transport water and nutrients throughout the plant. 16.1 WHAT ARE PLANTS? 323

Plant evolution Plants and green algae Because are similar in many ways to green algae, scientists think that both may have originated from an ancient species of green algae. Algae and green both have a life cycle that involves alternation of generations. Both contain the same type of chlorophyll and make the same type of starch. Also, both have similar cell walls. A brief evolutionary history The first ancestors of show up in the fossil record during the late Ordovician Period about 450 million years ago. Plants started out living in water, an ideal environment that supported cells and transported nutrients to the cells. As Earth s environments changed, had to adapt to life on land. They evolved adaptations for support, protection, and to prevent them from drying out. They also evolved vascular tissues for transporting water and nutrients throughout their bodies. Figure 16.2 shows the evolutionary relationships among plant groups. Figure 16.2: A cladogram that shows evolutionary relationships among major plant groups. 324

CHAPTER 16: PLANTS Non-vascular Characteristics of non-vascular Liverworts Mosses The non-vascular include the mosses and liverworts. These are small, simple usually found in moist locations. Because they lack vascular tissues, each cell in the plant must absorb water and nutrients through osmosis and diffusion. Thus, mosses and liverworts do not grow very tall. Mosses and liverworts need water to carry the sperm to the eggs for fertilization. The life cycle of nonvascular shows an alternation of generations. It includes a sporophyte stage that produces spores and a gametophyte stage that produces sex cells. You may have seen liverworts growing on wet rocks and soil in shady places (Figure 16.3). The sporophyte stage of a liverwort looks like a tiny palm tree. The body of the gametophyte stage is leafy and flattened. Rhizoids are root-like growths that extend from beneath the body and anchor the plant. They are not considered roots because they do not have vascular tissues. Mosses usually grow together in large colonies and cover an area like a carpet (Figure 16.4). Each moss plant consists of a leafy stalk with rhizoids at the base to anchor the plant. The sporophyte stage of a moss has a capsule on top that contains the spores. Figure 16.3: Liverworts. rhizoids - root-like growths on mosses and liverworts that anchor the plant to a surface and do not have vascular tissues. Figure 16.4: A carpet of moss. 16.1 WHAT ARE PLANTS? 325

Seedless vascular What are seedless vascular? Ferns The seedless vascular include ferns, club mosses, and horsetails (Figure 16.5). Because they have vascular tissues, these can grow taller than mosses and liverworts. A typical fern can reach heights of a meter or taller. Tropical tree ferns can reach a height of about 20 m. Ancestors of seedless vascular were even taller than their modern descendents. The first forests contained club mosses that grew to around 40 m tall! Modern club mosses are less than a meter tall. You can find ferns in tropical forests, temperate forests, and even in the Arctic. The form of a fern you will notice is the sporophyte. Figure 16.6 shows the structures of a fern. The leafy branch of the fern is called a frond. If you look underneath a fern frond, you may see small patches that contain the spores. Not every frond has spores under it. Ferns have an underground stem called a rhizome from which the fronds unfurl. Young fronds are tightly coiled and are called fiddleheads. The fern gametophyte is heart-shaped and about half the size of a pea. It has female parts that produce eggs and male parts that produce sperm. Like non-vascular, ferns need water to transport sperm cells to egg cells. Figure 16.5: Seedless vascular. Figure 16.6: The structures of a typical fern. 326

CHAPTER 16: PLANTS Vascular with seeds What are seeds? Gymnosperms and angiosperms The types of you are probably most familiar with are trees, grasses, and flowers. These familiar are very different from mosses and ferns. They have the ability to produce seeds. A seed is a structure that contains a plant embryo and a supply of food inside a protective covering. A seed forms after fertilization and is made up of a plant embryo, stored food, and a tough covering. The three parts of a seed are shown in Figure 16.7. Gymnosperms are a group of vascular whose seeds are not surrounded by a fruit. The seeds of many gymnosperms are housed in cones. Most gymnosperms are trees such as pine, fir, and spruce. Angiosperms, also known as flowering, produce seeds within a fruit. They are the most diverse of all plant groups and include fruit trees, roses, corn, grass, and oak trees. seed - a structure that contains a plant embryo and a supply of food inside a protective covering. gymnosperms - vascular, seedproducing whose seeds are not enclosed in a fruit. angiosperms - vascular, seedproducing whose seeds are enclosed in a fruit. Adaptations for life on land Seed have many adaptations for living on land. Seeds are more resistant to drying out than spores. Unlike spores, seeds contain stored food to nourish the embryo and help it sprout and grow. Also, seed do not require water for reproduction. Recall that mosses and ferns need water for fertilization to occur. In addition, seed have well-developed vascular systems for transporting water and nutrients throughout their bodies. Figure 16.7: The three parts of a seed. 16.1 WHAT ARE PLANTS? 327

More about seed Gymnosperms Conifers are a group of gymnosperms Angiosperms Gymnosperms do not produce flowers and their seeds are not enclosed in a fruit. Gymnosperms include conifers, cycads, and gingkoes. A gymnosperm called the bristlecone pine may be the oldest living organism on Earth (Figure 16.8). One bristlecone pine is believed to be almost 5,000 years old! The conifers, including pines and firs, are a group of gymnosperms that have cones. There are male and female cones on the same plant (Figure 16.9). Male cones produce male gametophytes called pollen. Pollen are dust-like particles that produce sperm. The female cone produces the eggs. Wind carries pollen to the female cone on the same or different. Sperm are released and fertilize the eggs. The seeds develop inside of the female cone. Angiosperms are flowering that produce seeds enclosed in a fruit. Angiosperms are divided into two classes monocots and dicots. The two classes have different numbers of cotyledons in their seeds. A cotyledon is an embryonic leaf found inside of a seed. Monocots (mono = 1) have one cotyledon and dicots (di = two) have two. In monocots, bundles of vascular tissue are scattered while in dicots, the bundles form a ring. Figure 16.8: The bristlecone pine is the oldest living thing on Earth. Figure 16.9: Male and female cones on a pine tree. 328